The pressure responsive system of the present invention utilizes a piezoresistive pressure responsive means that is normalized at a selected pressure level and for a selected temperature. The temperature of the transducer is maintained at the selected temperature by controlling the ambient temperature within the supporting housing for the transducer. A voltage output from the transducer, that varies with pressure, is applied to a gain stage having an output applied to a function generator that provides a signal varying with pressure. Thus, the need for a direct mechanical coupling between the pressure transducer and a signal generator is eliminated, thereby doing away with a complicated linkage or pick-off operating through a seal. Further, the piezoresistive pressure transducer requires only a minimum amount of power to provide a signal varying with pressure change for amplification in the gain stage.
Heretofore, various devices and systems for generating a pressure dependent signal have been provided. However, many of the systems furnishing such signals have been a complicated and expensive construction and accordingly are not suitable for small general utility applications, such as general aviation aircraft. Many such systems are difficult to install, thereby making servicing difficult and sacrificing reliability. On the other hand, prior art systems of a simple construction have not provided a satisfactory method of responding to a change in pressure, e.g., a change in aircraft altitude, to provide an accurate pressure related signal.
One application of the present invention is in aircraft control, such as an altimeter or altitude hold system. Altitude pressure responsive systems of the type to which the present invention relates usually consist of a housing containing pressure responsive means expansible and contractible in accordance with changes in ambient atmosphere, and a signal generator operable thereby. Some provision is made between the pressure responsive means and the generator to seal the latter from the ambient atmosphere affecting the pressure responsive means. One manner of sealing consists in the provision of a metal bellows rockable by the pressure responsive means through a linkage arrangement located at one side of the bellows to transmit such motion to the generator through a second linkage arrangement located on the other side of the bellows. Such a linkage arrangement has been found undesirable because the linkages include an undue number of moving parts which are subject to wear and consequently affect the operation of the system.
Other altitude pressure responsive systems have a chamber for the pressure responsive means which is separated and sealed from a second chamber containing the signal generator and other operating parts. Although this eliminates the need of a mechanical connection, the signal generator must operate through the seal to maintain isolation from the chamber containing the pressure responsive means. Such an alternative is likewise undesirable because of the loose coupling required between parts of the signal generator on opposite sides of the seal. This further increases the power requirements of the device.
In accordance with the present invention there is provided a system for normalizing the operating characteristics of a semiconductor pressure transducer that includes means for setting the zero output voltage of the transducer. The output signal of the transducer is normalized for a selected pressure level. The input impedance of the transducer is normalized for a selected temperature.
Also in accordance with the present invention, the selected temperature of the transducer is stabilized by apparatus that includes a housing for supporting the transducer. A heater is mounted in the housing to maintain the temperature of the transducer at a selected level. A circuit connected to the semiconductor transducer generates a temperature error signal applied to a controller that provides an output that is applied to the heater.
Further in accordance with the present invention, a controller responds to the temperature error signal to generate an output applied to the heater.
Also in accordance with the present invention, a pressure responsive system includes a transducer providing an output to a stabilized DC amplifier circuit that comprises an amplifier having a mixer connected to the input thereof. A switch having a first position connects the mixer to a reference source and the output of the amplifier to a sample and hold in a second position, the switch connects the mixer to an input signal from the transducer and connects the sample and hold to the mixer in addition to disconnecting the sample and hold from the output of the amplifier.